Put the sugar in a mixing bowl with the melted butter and egg yolks. Mix until dough is well mounted. Add the zest and orange juice and stir in gradually the flour and baking powder.
Mount whites until stiff and add to the mixture.
Coat the pan with parchment paper and then pour the mixture.

Cover and cook 35 minutes in the oven, until, by controlling the cooking with a knife, the blade does not come out clean.
Turn out onto a plate and decorate the cake with orange slices and a sprinkling of cinnamon to taste.